Parts made of composite materials are used in a variety of industries, including the aircraft industry. Some composite parts, such as inlets of aircraft nacelles, are perforated to aid in noise attenuation or dampening of aircraft engine noise. For example, some aircraft nacelles have approximately 800,000 holes per acoustic panel. These holes or perforations are formed into the composite parts using a variety of methods.
One method of perforation uses pin mats and rollers in a perforation process to mold holes into the composite part during cure. That is, an uncured composite skin is applied against a pinmat, similar to a bed of nails, and rollers press the composite skin toward the pinmat, forcing the pins into the composite part prior to cure. This method is costly and time consuming, requiring pin mat molding and contouring for specific composite part shapes, as well as a number of other steps that must be performed by hand by an operator. Furthermore, this method can create various defects on the composite part due to pin mat flaws, such as bent pins, missing pins, resin richness, seams, and a surface waviness anomaly associated with a transition from a perforated to a non-perforated area.
Another method of perforation involves a laser drilling process. However, laser drilling creates a heat-affected zone on the composite panel which can weaken the structural integrity of the composite part.
Yet another method of perforation involves perforating the composite part after cure using a mechanical drilling material removal process. However, this method involves a long set up and drilling time, and costly drill bits and other associated equipment. Furthermore, delamination forces may inadvertently separate layers of the composite part, damaging the integrity of the composite part.
Finally, another method involves perforating the composite part after cure using grit blasting while a perforated maskant (e.g., a stencil) is applied to a surface of the composite part to be perforated. The maskant may be made of rubber and protects the surface of the composite part and allows erosion of only the intended holes. However, this maskant is expensive and time consuming to produce, apply, and remove. Additionally, a large percentage of the abrasive particles thrown at the surface of the maskant are wasted in this grit blasting process, since the holes typically account for 14% or less of the total surface area of the composite part surface. Furthermore, the abrasive particles that rebound off the maskant can interfere with the effectiveness of an incoming abrasive particle stream. Finally, the amount of force used on the particles may be limited, so as not to destroy the maskant, but the less force used, the longer it will take to grit blast the perforations.